Add external worker supervision

[superposition]

Closes #98

Summary

• add an explicit external worker spec and supervisor for child-process start, status, stop, and restart-on-failure metadata
• add leash worker run -- ... to run a configured worker, print JSON status, then stop and reap it
• add scripts/smoke-worker.sh and wire it into scripts/smoke-all.sh
• document the worker surface and mark the roadmap item complete

Validation

• cargo test worker::
• bash -n scripts/smoke-worker.sh scripts/smoke-all.sh
• bash scripts/smoke-worker.sh
• cargo fmt --check
• cargo run --features mcp --bin leash-schema -- --check
• git diff --check
• cargo check --all-features
• cargo test --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --no-default-features --features sim,mcp
• scripts/smoke-all.sh
• cargo run --quiet -- worker run --name smoke-worker --hold-ms 50 -- node -e 'setInterval(() => {}, 1000)'\n- cargo package --locked --allow-dirty\n- cargo package --locked